The present invention relates to a medium in the shape of a disk, a tape or a card, such as an information recording medium where identification data is irreversibly recorded, and also to a recording/reproducing apparatus for reading the identification data from the information recording medium.
A magneto-optical disk can store a large volume of information with a portable size, and application of such a magneto-optical disk has been examined as a core recording medium in the rapidly developing multi-media. Since mass information can be recorded in a magneto-optical disk, management of information recorded therein, for example, prevention of illegal usage of the information has become very significant.
The present applicant has proposed a method of protecting information by using a medium identification code peculiar to each optical disk in Japanese Patent Application Laid-Open No. 5-257816 (1993). In this publication, however, a method of forming the medium identification code is not described, and the illegal usage of information cannot be actually prevented.
In order to definitely prevent the illegal usage of information, the present applicant has described a method of forming the medium identification code in Japanese Patent Application Laid-Open No. 9-73680 (1997). In this method, a medium identification code with a predetermined number of bits is irreversibly, namely, unrewritably recorded in a magneto-optical disk. An erasing operation is conducted on an area where the medium identification code is recorded, so as to judge whether or not a magneto-optical disk in use is a genuine product. When the medium identification code is not erased, the magneto-optical disk is genuine, and when it is erased, the magneto-optical disk is a copied product, and hence, information recorded therein is prohibited to be used.
The illegal usage of information can be thus prevented. However, a reproduced signal obtained from the irreversibly recorded medium identification code has such a small signal amplitude that there can be a case where the signal cannot be identified as the medium identification signal. As a countermeasure, the present applicant has described, in Japanese Patent Application No. 8-278466 (1996), an information recording medium where identification data is recorded by using an arrangement of irreversibly recorded unit areas and non-irreversibly recorded unit areas. In reading the identification data from this information recording medium, binary data is not read by using a difference of a signal amplitude but it is judged whether or not a read area is an irreversibly recorded area.
In reading such identification data, when a reproduced signal obtained from an irreversibly recorded area has a value approximate to a threshold value of binary data, an error in binarization can disadvantageously lead to a mistake in the judgement of the area.